A lightning arrester is a device which is placed between a phase and ground in a high tension line, and which serves to limit the amplitude and the duration of atmospheric over-voltages (surges due to lightning and to induction phenomena in the conductors), or to temporary electric overvoltages on the grid (operating surges).
The functions of a lightning arrester are firstly to withstand normal operating tension or a permanent basis, and secondly to pass the high discharge current which appears during a temporary surge, thereby protecting line apparatuses (transformers, . . . ).
These functions are generally provided by a core made of a material of the varistor type and based, for example, on zinc oxide ZnO whose electrical resistivity is highly nonlinear as a function of applied voltage.
This nonlinear characteristic enables such a lightning arrester to pass:
a low current (e.g. about 0.5 mA/cm.sup.2) when the operating voltage is applied on a permanent basis to the lightning arrester which then presents a very high resistance, this current is essentially capacitive in origin since the relative permitivity of such varistors is very high; or
a high current which may be as high as several tens of kiloamps, when the applied voltage reaches a trigger threshold above which the resistance of the varistor becomes very low.
European patent application EP-A-0.196.370 describes a lightning arrester structure comprising a central core made of a varistor type material, two end fittings which are threaded and glued onto the ends of the side wall of said core, and two intermediate spring blades between the bases of said core and the end fittings; an insulating covering having fins is provided around the side wall of the assembly. Such a structure suffers from drawbacks since firstly it requires the ends of the core to be machined, which machining is likely to damage the ends by creating cracks or breaks, and secondly it requires the end fittings to be glued which may disturb their electrical contact with the ends of the core.
British patent application GB-A-2.073.965 also described a lightning arrester in which the central core comprises a plurality of stacked cylindrical pellets of varistor type material, with two end fittings coming into contact with the pellets by means of spring blades; this assembly is held together mechanically by a one-piece sheath of heat-shrink material. This method is difficult to implement and is therefore expensive.
Preferred implementations of the present invention simplify the manufacture of lightning arresters and reduce the cost thereof.